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IPv6 and “Teardrop” Attacks

Way back in 1997, CERT issued an advisory about “Teardrop” attacks (http://www.cert.org/advisories/CA-1997-28.html). These are attacks where overlapping IP fragments are sent by a malicious actor in an attempt to either A) gain access to a protected system by evading IDS, firewall, and/or platform-based defenses, or B) disrupt the capabilities of the target environment – ideally in several parts of the network but especially the target node (the destination IP interface) itself.

In the 1997 CERT advisory, I am sure the concern was IPv4 systems. In brief, CERT’s advice was to contact vendors for a solution, and that solution was almost always to upgrade the platform OS version – it seemed the vendors were able to patch implementations. Presumably, this was done by discarding fragments associated with a Teardrop attack (any set of fragments where any two overlap). Juniper, for example, added a user configurable switch on platforms to explicitly drop overlapping fragments.

Any node sending overlapping fragments make no sense at all, from a protocol standpoint. Yet my understanding is that some old IP stacks did sometimes send overlapping fragments, not as an attack but as a result of a sloppy or buggy IP implementation. Thus, in an effort to promote interoperability with these stacks, other vendors would accept overlapping fragments. This philosophy goes all the way back to Jon Postel who said “be conservative in what you do, be liberal in what you accept from others”.

On the downside, this flexibility provided an exploitable vulnerability to malicious actors. Mostly, the attack was more directed at OS platform bugs than the IP stack. Modern platforms for the most part have implementations hardened against Teardrop-style attacks on IPv4 or IPv6.

And while it is true that the IETF IPv6 specification (RFC 2460) does not explicitly prohibit overlapping fragments, most vendors implemented protections against those IPv6 fragments as a matter of best practice. Later (December 2009) RFC 5722 made it out-of-spec for source nodes to send overlapping fragments, and made it required for the reassembling node (the ultimate destination) to silently discard all fragments associated with a fragmented packet when any two fragments overlap. Fragments are silently discarded, as opposed to having the reassembling node send ICMPv6 error messages, to guard against reflection attacks (where the attacker uses a victim IPv6 address as the source of the offending packets), and because the sending node is already acting “fishy”, so best not to burden any system (including the originator) with processing ICMPv6 error messages.

There is also the issue of fragmentation attacks against “atomic fragments”. These are IPv6 packets that are not actually fragmented, but carry the IPv6 Fragmentation Extension Header (with “Fragment Offset” set to zero and the “M” bit not set). The reason for these to be allowed in the IPv6 specification is to support certain IPv6-to-IPv4 translation solutions. To protect the capability from exploitation, there is an IETF draft (“Processing of IPv6 “Atomic” Fragments”) discussing ways to defend against fragmentation-centric attacks on this kind of packet.

With the improvements made to stack implementations over time (in the case of IPv4), and the fact that most IPv6 implementations were always “tighter”, the Teardrop attack has become a spent force on the Internet – really only affecting older platforms. Thanks to RFC 5722, there is now official IETF guidance on how IPv6 stacks should be implemented in terms of handling overlapping fragments, and giving platform vendors the green light to discard these corrupted fragments aggressively.

It is a classic example of the “arms race” nature of IP security – whether for IPv4 or IPv6. The bad guys discover a new vulnerability and an exploit, and the good guys close it off and harden everything else related to the vulnerability they can think of. A good reminder that IPv6 security is only “better” than IPv4 security in a few ways, and secure environments really depend on rigorous deployment of security best practices and constant vigilance by an informed and talented security team.